The selective charging and deflection of droplets of ink produced by continuous inkjet printers is a technique used to mark a wide variety of substrates. Meanwhile, valve jet inkjet ink technology is a technique used to mark a wide variety of substrates. Ink is maintained under pressure behind a closed nozzle or valve. When the valve is opened for a short time, ink emerges as a drop and is propelled by the pressure towards the print surface or substrate. Drops are, therefore, only produced as required in what is known as Drop-on-Demand printing.
Prints produced using these methods must be durable, as well as having good adhesive and rub resistance properties. Such inks are commonly used on high speed beverage lines due to their fast dry time and their ability to produce legible marks on curved surfaces.
It is also desirable for such a formulation to have good thermal stability properties so that the physical properties (such as viscosity and particle size) do not change significantly either during the storage of ink or during operation in the printer. This thermal stability (in both hot and cold environments) is required to ensure reliable printer running so that acceptable print quality and on-substrate performance in all customer environments can be assured.
It is common for substrates used within the beverage industry to have a layer of condensation on their surface due to chilled products being filled in humid environments. As well as providing a legible and durable mark onto such surfaces the codes must also survive immersion in cold water (if the product is chilled) as well as exposure to humid environments (both hot and cold). However in regions where bottles are reused the print must be easily removed through a caustic wash process. Products which are to be marked with such an ink may be a variety of colours; therefore for a mark to be visible the use of an opaque colour may be desirable.
Although titanium dioxide is commonly used in inkjet formulations to provide a high level of opacity, the settling and abrasive nature associated with this pigment can lead to reliability issues in an inkjet printer. Settling and abrasion resistance have been considered in WO00/63305 through the use of orange pigments to achieve a coloured code with high opacity. However, the performance of the inks described within WO00/63305 on wet surfaces is poor and not suitable for use in a chilled beverage line. US2005/0090579 pertains to an opaque inkjet ink for marking onto dark coloured substrates such as returnable glass bottles through the use of pigments. In particular, this document describes the use of opaque organic pigments in conjunction with acidic, acrylic resins and polyamines. Although the problem of providing visible codes with good adhesion onto dark glass substrates is addressed, there is no mention of how to provide an ink that is stable with respect to the settling and abrasion issues outlined above.
The use of a polyamine in conjunction with an acidic resin and hollow microspheres for producing an ink suitable for marking substrates where the code may be subjected to humid environments and/or ice water or condensation is outlined in U.S. Pat. No. 5,596,027. The microspheres are also exemplified as opacifying agents. U.S. Pat. No. 5,596,027 does not describe methods which would enable its ink to have the fast dry times required in beverage production sites. The production of fast drying inkjet compositions based on polyamines, acidic resins and organic solvents is taught in WO99/43760. Although both dye-based and pigment-based colorants are considered, there is no teaching on how to achieve a coloured code with high opacity.
Other approaches to achieve good adhesion onto glass substrates (with and without a layer of condensation) as well as good water resistance have been described in the art. U.S. Pat. No. 5,693,127 seeks to solve the issue through the use of an alkoxysilane polyalkylene imine glass adhesion promoter in conjunction with an acrylic binder. U.S. Pat. No. 5,744,519 describes the use of epoxy resins with a hydroxyl or carboxyl functional polymer. The use of a flexible thermoplastic polyurethane is described in U.S. Pat. No. 5,652,286.
Accordingly, it would be desirable to provide an improved or comparable inkjet ink. In particular it would be desirable to provide a more or a comparably thermally stable inkjet ink which is suitable for producing durable codes on articles during processes commonly used within the beverage industry.
It is one object of the present invention to overcome or address the problems of prior art inks or to at least provide commercially useful alternatives thereto. It is an alternative and/or additional object to provide an inkjet ink which is more thermally stable and/or more adhesive and/or faster drying than known inkjet inks.